Condensers are widely used in the manufacturing, chemical and energy industry. The direct system air-cooled condenser is a special type of condensers, which generally operates under vacuum and without cooling liquid, i.e. the vaporous medium is condensed directly by means of a cooling air flow.
The air-cooled condensers usually consist of a number of finned tubes connected in parallel between an upper header and a lower header. Inside the finned tubes, vaporous medium, preferably steam flows in the direction of the lower header and the cooling air flows outside the finned tubes approximately perpendicularly to them. On the outer side of the finned tubes, in order to compensate the low heat transfer coefficient of the air, fins are formed for increasing the air side surface. The steam in the finned tubes is condensed by the cooling air flow, the condensate is collected by means of gravitation in the lower header, and it is drained and taken back to the process circuit usually by a pump. Since the air-cooled condensers operate under vacuum, air must be removed from the condenser when starting up.
It is well known that not only pure steam enters the finned tubes but also a very low quantity of non-condensible gases, mainly air. One part of the non-condensible gases is carried by the steam, while the larger part is getting into the steam as a result of the leaks of the process circuit. An example for the possible leak is the dividing plane of the steam turbine. This air quantity--since the air is a non-condensible gas--is concentrated in the finned tubes of the condenser and deteriorates the efficiency of the heat transfer, i.e. at a given temperature difference less heat is transferred. Therefore, the air must be removed continuously, which is usually carried out by a continuously running vacuum pump.
It is also known that the steam being condensed in the finned tubes moves continuously in a gradually decreasing quantity, and there is at least one point in the finned tubes where the velocity of the steam is zero. This at least one point is called congestion point. The congestion point is characterised in that the steam flows to it from all directions, but steam does not move away from it in any direction. The place of this congestion point depends on many factors, primarily on the geometry of the heat exchanger, on the velocity and temperature distribution of the cooling air flow, etc.
If the steam includes non-condensible gases, primarily air, in a very small quantity, for example approximately 0.01% in relation to the steam quantity, the air will be travelling exactly towards the congestion point described above. To make sure that air is not being concentrated at this point, thus to avoid the formation of the so-called air pocket, it must be removed continuously, i.e. the congestion point has to be displaced outside the finned tube. If this is not done, the following consequences have to be faced.
The air pocket expands gradually, thereby reducing the steam-side heat transfer coefficient. PA1 In cold weather it would result in the undercooling of the surface of the finned tube, which could cause freezing up of it. PA1 Finally, with the increasing partial pressure of the air, the air pocket would reduce the temperature of condensation and thereby the temperature difference between the two sides of the heat exchanger, i.e. the driving force of the heat exchange.
It decreases the effective inner surface of the finned tube by blocking the internal surface from the condensing steam.
The most suitable place for removing the air from the finned tubes is exactly the congestion point mentioned above. This would be simple if the place of the congestion point were constant in the finned tubes. Unfortunately, this is not the case, because this place could be different under various operating conditions. In addition, not only the change in operating conditions, but also the inevitable flow asymmetry would also make the place of the congestion point uncertain. To make sure that under all operating conditions and in all finned tubes the congestion point is at a determined place outside the finned tubes, a geometrical design must be implemented where the velocity and direction of the steam flow are determined and sufficiently high in the vicinity of air extraction. A general solution may not be identified, and a different approach is to be applied for each heat exchanger geometry.
It is obvious from the discussion above that through the air extraction, not only air but steam must also be extracted, because only in this way can it be ensured that the steam velocity is appropriate everywhere, i.e. that an air pocket is not developed at any point. One known solution is to introduce a large part of the steam quantity into the air extraction. The disadvantage in this case is that a significant heat quantity has to be removed from the steam-air mixture. Instead, another known solution is generally chosen according to which an after-cooler is connected after the so-called main condenser.
The after-cooler condenses a relatively large, generally 15 to 25% part of the steam, thereby ensuring the appropriate velocity and the determined flow directions at the air extraction. To make sure that the undercooling of the condensate is not excessive which is to be avoided from the aspect of efficiency and frost risk, the after-cooler is usually connected in counterflow, i.e. the condensate flows down on the wall of the after-cooler in an opposite direction to the steam flowing upwards with gradually increasing air concentration. At the end of the after-cooler, where most of the steam has already been condensed, the concentrated air-steam mixture is extracted usually by a vacuum pump.
In the case when there is more than one row of finned tubes in the direction of the cooling air flow, or there is only one row of finned tubes but they are divided into separate internal channels by separation walls, the first finned tube/channel row receives colder cooling air than the next rows in the direction of the cooling air flow, therefore it condenses the entering steam along a shorter path than the other finned tubes/channels. Therefore, a congestion point will be developing therein towards which steam from the other finned tubes/channels will flow upwards via the lower header. The same can be the case for the second etc. finned tube/channel rows, wherein the distance between the congestion points and the upper header in the successive finned tube/channel rows is gradually increasing. In case when there is air in the steam, it flows in the direction of the congestion point(s) mentioned above and after a while it fills the section(s) below the congestion point(s). This air pockets, as mentioned above, may result in the freezing up of the finned tubes in cold weather.
To eliminate these air pockets, according to a known solution, the congestion points are displaced in a way that a steam quantity sufficient to shift the congestion points in the first finned tube/channel to the lower header is removed through an extraction pipe connected to the lower header. This can be assumed like cutting across the heat exchanger at the congestion point in the first finned tube/channel, and the above quantity of the steam is transferred to an after-cooler. Such an air-cooled condenser is described in DE GM 78 12 373, according to which a separate after-cooler connected in series to the condenser is provided. This solution has several disadvantages. First of all, a separate after-cooler is to be designed. Secondly, the friction loss of the cooling system will be increased for two reasons. One is that the path to be travelled by the steam is longer. The steam flowing at a high velocity suffers higher pressure loss in the finned tubes, and as a consequence, along the finned tubes the temperature of the steam will be reduced, and so is the temperature difference between the steam and the cooling air, which difference is proportional to the efficiency of the heat exchanger. The other is that the heat exchanger surface reserved for the after-cooler connected in series to the condenser reduces the heat exchanger surface of the condenser, thereby reducing the steam entrance cross section.
This latter disadvantage can be avoided by a known solution described in WO 98/33028. According to this solution an air-cooled condenser having so-called integrated multichannel finned tubes is provided, which finned tubes can be produced by, for example, extrusion. The fins on the outer side of the finned tubes can be made from the tubes by machining or can be fixed by welding or soldering on the extruded tubes. The after-cooler is integrated into or separated from the multi-channel tubes in a way that in the appropriate channels in the vicinity of each already described congestion point, a closure element is arranged. Adjacent the closure elements there are breakthroughs formed in the separation walls of the channels, which direct the steam into neighboring channels. The breakthroughs also ensure that the condensate developed above the air pockets is drained into the lower header. The separation walls separating the after-cooler from the condenser section do not have any breakthroughs, and this guarantees the determined flow direction of the steam-air mixture towards an air extraction. In the separation walls of the after-cooler and in that of the condenser there are further breakthroughs for allowing free flow of the steam between their neighbouring channels. The advantage of this structure is that the integrated after-cooler does not decrease the incoming steam entrance cross section of the finned tubes, so the steam-side pressure drop is decreased. On the other hand, the path to be travelled by the steam is relatively long, and this is detrimental to the efficiency of the heat transfer.